Air-over-oil circuit

 Fig.8.22 illustrates a typical air-over-oil circuit to make best use of advantages of both fluid mediums for counterbalancing application.

AIR-OVER-OIL CIRCUIT

1. Circuit

Fig.8.22 illustrates a typical air-over-oil circuit to make best use of advantages of both fluid mediums for counterbalancing application. This circuit uses an air-oil surge tank, a manually operated 3/3 DC valve, a FRL unit, a flow control valve, a pressure relief valve, and a cylinder. In the surge tank, oil is filled at the bottom and the air at the top.

2. Operation

Extension: When the 3/3 DC valve is shifted to its upper envelope flow path configuration, the compressed air flows via FRL unit and the valve to the surge tank. So the surge tank is pressurized by the compressed air. This pushes the oil (out of the bottom of the surge tank) to the blind end of the cylinder through the flow control valve. Thus the cylinder extends to lift a load. Here the flow control valve can be used to regulate the extension speed of the cylinder.

Retraction: When the 3/3 DC valve is shifted to its lower envelope flow path configuration, the air leaves the surge tank and exhausts into the atmosphere via the DC valve. So the oil at the blind end of the cylinder returns back steadily through the flow control valve and thus the cylinder retracts.

The load (Fload) can be stopped at any intermediated position by shifting the 3/3 DC valve to its spring-centered position. This circuit eliminates the use of a costly hydraulic pump and an oil reservoir. Also this circuit uses the oil for the advantage of generating high force and precision control of the cylinder.